Electrokinetic strength enhancement of concrete

ABSTRACT

A method and apparatus for strengthening cementitious concrete by placing a nanoparticle carrier liquid in contact with a first surface of a concrete section and inducing a current across the concrete section at sufficient magnitude and for sufficient time that nanoparticles in the nanoparticle carrier liquid migrate through a significant depth of the concrete section.

This application claims the benefit under 35 U.S.C. 119(e) of U.S.provisional application Ser. No. 60/647,509 filed on Jan. 27, 2005.

This invention was developed in part with funds from contractNCC5-573-NASA/LEQSF(2004)-DART-10 awarded by NASA and the U.S.Government may have certain rights to the invention as provided by thatcontract.

FIELD OF INVENTION

The present invention relates to methods and apparatuses for improvingthe characteristics of concrete. One embodiment of the inventionincludes a method and apparatus for increasing the strength of concreteby using an electrical field to move particles into pores within theconcrete.

BACKGROUND OF INVENTION

The movement of particles into porous materials such as concrete involveseveral processes, including adsorption, liquid diffusion, capillaryabsorbtion, bulk laminar flow, and electrokinetic transport.Electrokinetic transport is the phenomenon of charged particles movingin response to an applied electric field. Electrokinetic transportincludes ionic conduction, electrophoresis, and electroosmosis. Ionicsolution conductivity accounts for the overwhelming majority ofconductivity measured in cement based materials. In an aqueous system(cement concrete structures generally retain a certain moisture contentin most conditions), ions can be induced to drift in response to anapplied electronic field. Electrophoresis is characterized by themovement of a solid particle dispersed in an electrolyte under theinfluence of an electric field. Electroosmosis is the induced flow ofwater through a porous medium such as sand, clay or concrete when anelectric potential is applied across the medium.

Depending on the degree of saturation of a concrete sample, any or allof the above transport processes may occur and a number of structuralfactors may influence the transport processes. Concrete is a mixture ofsand, stone (or other aggregate) flued together with a hardened cementpaste that is porous. This pore structure is the dominantmicrostructural feature governing transport. Pore structure originatesfrom the microstructure when water, anhydrous cement grains, andaggregate are mixed. Capillary pore structure initially assumes theshape of the space occupied by mix water. However, hydration of thecement yields calcium silicate hydrate (C—S—H) the primary binder inhardened cement paste. The capillary pore structure is developed asthese hydration products form. Capillary pores tend to dominatetransport processes and specific structural characteristics of capillarypores which influence transport include pore volume of the sample, sizedistribution, tortuosity, and connectivity. The aggregate present in theconcrete may influence transport in different ways. Low porosityaggregate tends to impeded mass transport by blocking more direct pathsthrough the hardened cement paste pores. Conversely, there can be highporosity at the paste-aggregate interfacial zones. Microcracks and bleedpaths also influence particle transport. Microcracks form during dryingof the calcium silicate hydrate layers which shrink and lead to tensilestress and cracking. Tensile stress do to plastic shrinkage, stressesfrom applied loads, thermal expansion or freezing pore water may alsoinducing microcracking. Bleed paths occur when prior to setting, wateraccumulates around aggregate and moves toward the surface of the cementpaste. Discrete flows can join together to form bleed paths which remainafter setting of the cement paste.

Changes in water content of hardened cement pastes have significantimpacts on transport mechanisms and rates. At relative humidities above45%, evaporable pore water is said to exist. Above this threshold, whilethe permeability of gases is increasingly blocked by liquid waterbarriers, the transport of aqueous ions or particles progresses morerapidly as the presence of evaporable capillary water increases. Thus,water content is an important factor affecting electrokinetic transportin concrete.

BRIEF DESCRIPTION OF INVENTION

Preferred embodiments of this invention include a method and apparatusfor strengthening cementitious concrete by placing a nanoparticlecarrier liquid in contact with at least a first surface of a concretesection and inducing a current across the concrete section at sufficientmagnitude and for sufficient time that nanoparticles in the nanoparticlecarrier liquid migrate through a significant depth of the concretesection. These particles react with calcium ions liberated from residentcalcium hydroxide to form strong phases that increase the strength ofthe concrete.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates one embodiment of the invention applied to a sectionof concrete material.

FIG. 2 illustrates another embodiment of the invention wherein the rebarserves as an electrode.

FIG. 3 illustrates an embodiment of the invention applied to uncuredconcrete.

FIG. 4 illustrates an embodiment of the invention employing a spongematerial to assist in bringing particles into wet electrical contactwith the concrete surface.

FIG. 5 illustrates a method to treat a column using one embodiment ofthe present invention.

FIG. 6 illustrates a masonry block treated by one embodiment of thepresent invention.

FIG. 7 illustrates a stress vs. radial distance plot for an untreatedconcrete section.

FIG. 8 illustrates a stress vs. radial distance plot for a treatedconcrete section.

FIG. 9 illustrates cross-section diagrams showing crack penetration.

DETAILED DESCRIPTION OF SELECTED EMBODIMENTS

FIG. 1 illustrates one embodiment of the present invention, an apparatus1 employed for increasing the strength of a concrete section 3. As seenin the cross-sectional view of FIG. 1, concrete section 3 will have twocontainers 9 positioned on opposing surfaces 20A and 20B. Containers 9may be any substantially liquid impermeable enclosures which will retaina liquid while allowing the liquid to contact the respective concretesection surfaces 20A and 20B. Any type of conventional (or futuredeveloped) seal 10 may be employed along the edges where containers 9contact surfaces 20A and 20B in order to prevent liquid from escapingcontainers 9 along such edges. The dimensions of containers 9 willgenerally be sufficient to substantially cover the surface area ofconcrete section 3 to be treated. Positioned within each container 9will be an electrode 6 and a conductive liquid. The electrodes 6 will beconnected to an electrical power source 11.

In the embodiment seen in FIG. 1, the left electrode 6 will be anode 7and the right electrode 6 will be cathode 8. Likewise, the conductiveliquid in the left container 9 will be a nanoparticle carrier liquid 4and the conductive liquid in the right container 9 will be anelectrolyte solution 5. In this embodiment, the nanoparticle carrierliquid 4 is a colloidal alumina suspension such as product no. 8676provided by Nalco Chemical Corp. of Chicago, Ill., which comprises 10%by weight of 2 nm alumina particles in a chloride stabilized carrierliquid. The electrolyte solution 5 in this embodiment is a saturatedcalcium hydroxide (CaOH)₂ solution that is less than 1 molar. However,the nanoparticle carrier liquid 4 may any liquid containing particleswhich tend to strengthen concrete when moved into the pore spaces ormicrocracks existing in concrete. As used herein, nanoparticle generallymeans a particle that is less than 1 micron in size. A nanoparticlecarrier liquid is generally a liquid containing a substantial percentageof nanoparticles, but such carrier liquids may also include somepercentage of particles larger than 1 micron. In many embodiments, thenanoparticle carrier liquid will be a colloidal suspension carryingnanoparticles of compounds such as alumina, silica, sodium silicate, orother silicates and aluminates. Other embodiments may includenanoparticles of latex, or polymer particles. However, the carrierliquid is not necessarily limited to suspensions and solutions sincevapors and air could be could be carrier fluids. Typical weight percentconcentrations of nanoparticles could be about 1 to about 60 weightpercent, about 10 to about 50 weight percent, about 30 to about 50weight percent, or any range between 1 and 60 weight percent. Likewise,electrolyte solution 5 could be any number of conductive suspensions orsolutions such as calcium hydroxide, tap water, rain water, and purewater that has been allowed to absorb conductive ions from itssurroundings (e.g., water in contact with the concrete). However,electrolyte solutions containing aggressive species such as chlorides orsulfates are not typically preferred.

Electrodes 6 may be formed of any material which is sufficientlyconductive to carry out the functions of the present invention.Exemplary electrode materials include zinc, cadmium, steel, aluminum,copper, monel, or other conductive metals or conductive-oxide-coatedversions of these metals. Post-tensioned steel and other applicationsthat may be susceptible to hydrogen embrittlement hazards are lesspreferred for Electrodes 6. In many embodiments, it is advantageous toform the anode 7 from a relatively corrosion resistant but conductivematerial such as graphite, a conductive polymer, or a mixed-metal-oxidetitanium alloy.

In the embodiment of FIG. 1, the conductive liquids in containers 9 actto provide a uniform electric field across the portion of surfaces 20Aand 20B which are in contact with the liquids. The electric field willinduce movement of charged particles in the carrier liquid toward theelectrode having the opposite charge of the particle and thus into theconcrete section 3. Many factors affect the velocity at which particlesmove into and within the concrete, including the particle size, particlecharge, pore fluid ion concentration, pore wall and particle zetapotential, pore fluid viscosity, electroosmotic coefficient ofpermeability, fluid pressure acting in opposition to particle transport,thickness of concrete, the size and number of micro-cracks, the porosity(both pore volume and tortuosity) of the concrete, the moisture contentof the concrete and the strength of the electric field. If the electricfield strength is stated in terms of current density, it may vary fromas little as 10 mA/ft² (or less) to more than 1000 mA/ft². The timenecessary for nanoparticles to disperse though the entire thickness ofconcrete section 6 is of course based upon the thickness of the sectionand the nanoparticle velocity.

As mentioned above, one nanoparticle carrier is colloidal alumina.Alumina may be consider a “pozzolan,” which may be defined as asiliceous, aluminous or siliceous and aluminous material which in itselfpossesses little or no cementing property, but will in a finely dividedform and in the presence of moisture chemically react with calciumhydroxide at ordinary temperatures to form compounds possessingcementitious properties. In regards to alumina, a pozzolanic reaction,may be described as the aluminous pozzolans reacting with the(non-cementitious) calcium hydroxide in the hydrated cement paste toproduce (highly cementitous) calcium aluminate hydrates which yieldhigher strength and dramatically reduced the permeability of theconcrete. This reaction may be represented by the formula:CH+A+H→C-A-H;where CH is Ca(OH)₂, A is Al₂O₃, H is H₂O, and C-A-H is calciumaluminate hydrate. Where the pozzolan is silica, a similar pozzolanicreaction may be described as:CH+S+H→C—S—H;where S is SiO₂ and C—S—H is calcium silicate hydrate.

Sodium silicate may be another nanoparticle suspension utilized incertain embodiments of the present invention. Sodium silicate is any oneof several compounds containing sodium oxide, Na₂O, and silica, Si₂O, ora mixture of sodium silicates with varying ratios of SiO₂ to Na₂O,solids contents, and viscosity. Traditionally, sodium silicates areclassified according to the acid from which they are derived asOrthosilicate Na₄SiO₄; Metasilicate Na₂SiO₃; Disilicate Na₂Si₂O₅;Tetrasilicate Na₂Si₄O₉. The sodium silicate species can change from onetype to another depending upon conditions and the relativeconcentrations of each one that is present. This process may providemore of the actual species undergoing the reaction. Sodium silicate (inthe case of Metasilicate) enters pores and combines with calciumhydroxide to form C—S—H gel as follows:Na₂SiO₃ +yH₂O+xCa(OH)₂ →xCa.SiO₂ .yH₂O+2NaOHAs used herein, “nanoparticle carrier liquid” includes (but is notlimited to) any liquid containing nanoparticles (including ions) insuspension, solution, or dissolved, which react with other components toform strong phases to increase the strength of concrete. In certainembodiments, the nanoparticles will be particles or ions which reactwith calcium hydroxide (either resident in or driven into the concrete)in order to form this stronger phase.

In addition to pozzolans combining with calcium hydroxide originating inthe cement section itself, certain embodiments of the present inventionemploy Ca(OH)₂ in the electrolyte solution 5 to increase theavailability of Ca⁺⁺ ions. For example, if colloidal silica is thenanoparticle carrier 4 in FIG. 1, it will be understood that locationsof anode 7 and cathode 8 would be reversed in order to move thenegatively charged silica particles into concrete section 3. Likewise,the positive Ca⁺⁺ ions in the electrolyte solution 5 would be moved intoconcrete section 3, where such Ca⁺⁺ would be available for thepozzolanic reaction described above. Although this polarity arrangementmay tend to remove OH⁻ ions from the concrete, the abundance of OH⁻ inthe concrete means any resulting removal of OH⁻ is inconsequential tothe overall treatment process.

Although FIG. 1 suggests containers 9 are enclosed (i.e., additionalliquid is not shown being added), this need not always be the case. Insome embodiments, the volume of nanoparticle liquid in container 9 willhave sufficient nanoparticles to completely treat the concrete section3. However in other embodiments, additional nanoparticle carrier liquidmay be fed into container 9 (i.e., exchanged with the depleted carrierliquid) if a greater mass of nanoparticles is needed for completetreatment.

Alternate embodiments of present invention applying similarelectrokinetic transport principles as described above may be seen inFIGS. 2-5. FIG. 2 represents a concrete section 3 having reinforcingsteel or “rebar” 13. A container 9 will be formed around one end ofconcrete section 3 such that the nanoparticle carrier liquid 4 is incontact with the surface 21. A wire, mesh, or flat plate electrode 12will be positioned in the nanoparticle carrier liquid 4 and will extendsubstantially the length of concrete section 3 (i.e., the length runningperpendicular to the cross-sectional plane shown in FIG. 2). In thisembodiment, the rebar 13 will be connected to current source and serveas an electrode (e.g., cathode 8). When plate electrode 12 forms theanode and rebar 13 forms the cathode, positively charged nanoparticles(e.g., alumina) in carrier liquid 4 will be driven toward rebar 13 bythe electric field established between the electrodes. Clearly, thedistance which must be traveled by the nanoparticles is greater in FIG.2 than FIG. 1, thereby suggesting the use of a higher current density ora longer treatment duration than might be necessary for the arrangementof FIG. 1.

Although the embodiments of FIGS. 1 and 2 contemplate the treatment of arigid cured concrete section 3, FIG. 3 illustrates an alternateembodiment could be employed to treat an uncured concrete section 15. Aform 16 will be provided which, as is well know in conventional concreteproduction, will contain the uncured flowable concrete mix in theintended shape of the ultimate concrete section. The sides of form 16will be higher than the depth of the concrete section in order toaccommodate a fluid (explained below) overlying the placed concrete mix.A wire mesh electrode 14A will be positioned in the bottom of form 16prior to the pouring of any concrete mix. Wire mesh electrode 14A can beof various types of mesh, preferably with a large enough opening toallow water to pass readily through and a small enough opening to ensurethe distribution of a uniform electric field across the substrate. Inone embodiment, a mesh with a 1/16 inch opening size may be placed indirect contact with the surface. In another embodiment, a large meshwith and opening size of 12 inches may be located 6 inches from theconcrete surface. Such meshes will generally conform to the bottomdimensions (width and length) of the form 16. However, alternateembodiments of wire mesh electrode 14A could be greater or less than thebottom dimensions of form 16. Likewise, in other embodiments notspecifically illustrated, rather than a mesh electrode, a conductiveplate could be positioned in the bottom of form 16. Alternatively, anyreinforcing steel intended for use in the concrete section 15 couldserve as the electrode. After placement of electrode 14A, a conventionalcement paste mix (e.g., water, cement, course and fine aggregate) willbe placed in form 16 in an amount sufficient to produce the desiredthickness of the concrete section. Optionally, the mix design may beadjusted with more water and less cement powder to optimize cost. Asecond mesh electrode 14B is spaced over the top of the uncured cementmix. In the embodiment shown, the mesh electrode 14B will be spacedslightly above (e.g. at least 0.5 cm) the top of the placed concretemix. However, in other embodiments, the mesh can also be touching theconcrete or be positioned just beneath the concrete surface. Ananoparticle carrier liquid 4 is then carefully introduced onto of theconcrete mix in a manner that does not disturb the concrete mix and inan amount sufficient to immerse the mesh electrode 14B, assuming it isnot cast in. Thereafter, an electric current is applied between theelectrodes 14A and 14B in order to induce the migration of nanoparticlesthrough the concrete section. In the embodiment shown, a current densityof about 10 mA/ft² to about 1000 mA/ft² could be employed (althoughcurrent densities outside this range are possible) and in one particularexample, the current density would be about 500 mA/ft². The duration ofcurrent application will depend on factors such as the desired depth towhich nanoparticles are being directed, the magnitude of the currentdensity, and the water content of the cement mix. In many embodiments,the duration should be sufficient to move particles into all thelocations that are subject to the electric field, since application ofan electric field to the concrete without nanoparticle strengthening mayresult in a weakening of the portion of the concrete. If the treatmentoccurs over several days, the concrete may be more than sufficientlycured to remove from form 16 and pull wire mesh 14A from the bottom ofthe concrete section. Although the method of FIG. 3 has been describedas being initiated with uncured concrete mix, the method could beapplied to any hardened cement paste (i.e., the concrete having reachedthe binder phase that holds all the stone and sand in a fixed matrix).

A further embodiment seen in FIG. 4 utilizes a different method forplacing the nanoparticle carrier liquid into contact with the concretesection 3 (a cured concrete section in the example of FIG. 4). Placedagainst at least one surface of concrete section 3 will be a flexibleporous material 17 which is capable of at least partially absorbing andretaining a nanoparticle carrier liquid brought into contact with theflexible porous material 17. In one example, flexible porous material 17is a sponge material. Such sponge materials could include naturalsponges, e.g., an elastic porous mass of interlacing horny fibers thatforms the internal skeleton of various marine animals and is able whenwetted to absorb water; or synthetic sponges, e.g., a porous rubber orcellulose product having properties similar to a natural sponge.Flexible porous material 17 will typically cover the general area ofconcrete section 3 into which nanoparticles are to be introduced. Thethickness of flexible porous material 17 may vary in differentembodiments. In one example, flexible porous material 17 may beapproximately 2.5-7.6 cm thick. However, in other embodiments, theporous material need only be thick and flexible enough to accommodatemost of the surface topography of the substrate being treated so thatwet electrical contact is maintained during treatment. FIG. 4 alsoillustrates a reservoir 18 of nanoparticle carrier liquid 4 whichcommunicates with flexible porous material 17 via supply line 19. Thetransfer of fluid could be accomplished through a gravity feed system assuggested in FIG. 4 or though some type of pumping arrangement.Reservoir 18 will replenish the nanoparticle carrier liquid 4 inflexible porous material 17 as the liquid evaporates and asnanoparticles are driven from flexible porous material 17 into concretesection 3. A mesh electrode 14B will be positioned over flexible porousmaterial 17. On the surface of concrete section 3 opposite meshelectrode 14B, another flexible porous material will be positioned tobring an electrolyte solution into wet electrical contact with thatsurface of concrete section 3. The mesh electrode 14A will be positionedon the flexible porous material. Although not explicitly shown in FIG.4, certain embodiments could include a reservoir of electrolyte solutionto supply the flexible porous material 17. Likewise, the porous materialcould be positioned under a mesh electrode of conductive fabric, e.g., afabric containing a weave of flexible graphic wire. As in theembodiments described in FIGS. 1-3, application of a current betweenmesh electrodes 14A and 14B at a sufficient magnitude and for sufficientduration will induce nanoparticles to move into and through at least asignificant portion and preferably the entire depth of the concretesection 3.

Many variations of the method seen in FIG. 4 are within the scope of thepresent invention. The material 17 need not be flexible and there may beapplications where a comparatively rigid porous material may beemployed. Additionally, it may not always be necessary to have thematerial 17 re-supplied with liquids from some external source such asreservoir 18. Rather, in certain applications it may suffice to simplyexpose material 17 to nanoparticle and electrolyte liquids at theinitial stage of the treatment process and this will provide sufficientnanoparticles for the complete treatment of the concrete section.Alternatively, material 17 could be refreshed with liquids at one or twopoints in the treatment process (as opposed to continuous supply from areservoir). Although FIG. 4 illustrates separate mesh electrodes 14A and14B positioned over porous material 17, other embodiments might includemetal fibers (or other conductive materials) incorporated into porousmaterial 17, thereby combining the liquid retaining function and theelectrode function into a single section of material. Likewise, if rebaris present in the concrete section 3 (for example near the side on whichmesh electrode 14A rests), then the rebar may substitute for electrode14A (and eliminate the need for porous material 17 under electrode 14A).Also, porous material 17 could be applied to an uncure concrete mix toreplace the pool of carrier liquid described in the embodiment of FIG.3.

FIG. 5 illustrates another embodiment using a porous material to retaina nanoparticle carrier liquid in contact with a concrete section. In thecross-sectional view of FIG. 5, the concrete section is a concretecolumn section 25. As is typical with concrete columns, column section25 will include a series of rebar members 15. In FIG. 5, a continuoussection of porous material 26 is wrapped around column section 25 and amesh electrode 14 is either intermeshed or positioned atop porousmaterial 26. In this embodiment, one or more of rebar members 13 willact as the electrode 8. Typically, the center most rebar member(s) 13will act as electrode(s) 8 in order to move nanoparticles as far aspossible toward the center of column section 25 (i.e., nanoparticles arenot expected to migrate any further inward than the most central rebarposition of electrode 8). Although not shown, a nanoparticle carrierliquid reservoir could be connected to porous material 26. Additionally,porous material 26 need not be continuous around the circumference ofthe column, but could be placed in discrete sections to cover asubstantial portion of the column's circumference (with the same beingtrue for mesh electrode 14).

The foregoing specification has described only a few specificembodiments of the present invention and those skilled in the art willrecognize many alternatives and variations. As suggested above, it isnot necessary in every embodiment to treat (i.e., disperse nanoparticlesinto) the entire depth of the concrete section. However, in somesituations, application of a substantial current density across aconcrete section may result in weaker concrete in those portions intowhich nanoparticles do not extend. Additionally, when dealing withconcrete sections which have become quite dry (at least on the outerinch or two of the concrete surface), it may be advantageous tothoroughly wet the concrete surface prior to beginning theelectrokinetic treatment. Nor is the present invention limited toapplying the nanoparticles in the methods described in FIGS. 1-5. Forexample, it is envisioned that the nanoparticle carrier liquid could bea thick viscous liquid with a consistency similar to paint. The carrierliquid would then be “painted” onto the surface of the concrete sectionwith an electrode (e.g., a mesh electrode) placed directly on thepainted section of the cement. Alternatively, particles could be usedthat are transported through the air using a powder coating wand. Thesubstrate would preferrably be wet so that particles that absorb ontothe wall could continue to migrate in the electric field. Anotheralternative could involve the use of a conductive gel. The gel would beloaded with particles, applied to the concrete substrate and driven intothe concrete using and an electrode immersed in the gel, or thesubstrate. The gel may be vacuumed and recycled for a futureapplication. All such variations and modifications should be consideredwithin the scope of the claims.

EXAMPLES Example 1

Tests were conducted to determine the impact of the electrokinetictreatment on common heavy weight and light weight masonry blocks. Aheavy weight block is a hollow load-bearing concrete block8-by-8-by-16-inches nominal size, having two hollow sections, andweighing from 40 to 50 pounds when made with heavyweight aggregate, suchas sand, gravel, crushed stone, or air-cooled slag. The same size blockis considered light weight and weighs only 25 to 35 pounds when madewith coal cinders, expanded shale, clay, slag, volcanic cinders, orpumice. The masonry blocks were obtained from American Block Corporationof Bossier City, La. in July 2004. The light weight blocks were treatedand tested seven months following production. The heavy weight blockswere treated and tested 16 months following production. Prior totreatment the masonry blocks were fully saturated in a solution of 1molar calcium hydroxide. The electrokinetic treatment was carried outfor a period of five days.

The heavy weight and the light weight masonry blocks were stabilized insaturated calcium hydroxide solution until a time when the difference inweight of the masonry blocks on two consecutive days showed an increaseof less than 0.2% (in accordance with ASTM C 140). This was done toensure that the specimens were fully saturated during the five-daytreatment period. Saturation was established to minimize absorption ofchemicals through capillary draw. Allowing capillary draw would permit achange in weight that would not be due to electrokinetic treatment. Alack of saturation could also reduce treatment access to the porestructure and thus reduce the enhancement of load resistance andpermeability reduction that would otherwise be available. Electrokinetictreatment does not penetrate well into unsaturated spaces. This lack ofpenetration could reduce the enhancement of load resistance orpermeability reduction that could be obtained. In each case the masonryblocks were successfully stabilized within approximately 13-19 days.

Following moisture stabilization the specimens were prepared for afive-day treatment period as suggested in FIG. 6. Window putty was usedas a fluid barrier at the bottom of each specimen so that when thesodium silicate (Oxychem 50, 44 wt-% Na₂SiO₃) and calcium hydroxidesolutions were placed (see FIG. 6), this seal prevented the flow ofliquid beneath the block. This allowed enough time for staging theexperiment without premature mixture of the reactants. The negative poleof the power supply was connected to the steel mesh that was immersed inthe sodium silicate. The positive pole was connected to the graphiteelectrodes immersed in calcium hydroxide solution. The compression testwas conducted according to ASTM C 140 specifications to determine theload resistance of the specimens and for calculating the percentageincrease in load resistance after treatment.

The light weight block specimens were labeled as follows:

Set 1 contains Experimental Block 1 (LEB1) and Control Block 1 (LCB1),

Set 2 contains Experimental Block 2 (LEB2) and Control Block 2 (LCB2),and

Set 3 contains Experimental Block 3 (LEB3) and Control Block 3 (LCB3).

The heavy weight block specimens were labeled as follows:

Set 1 contains Experimental Block 1 (HEB1) and Control Block 1 (HCB1),

Set 2 contains Experimental Block 2 (HEB2) and Control Block 2 (HCB2),and

Set 3 contains Experimental Block 3 (HEB3) and Control Block 3 (HCB3).

The treatment circuit was set for constant voltage. For the light weightspecimens a current of 38 mA was applied for Set 1 and a current of 76mA was applied for Set 2 and Set 3. The heavy weight blocks were alltreated with a current of 76 mA. Some current and voltage drift wasobserved and was mainly due to electrode polarization. In addition, theloss of ions due to the C—S—H reaction caused resistance to go up andthus an increase in voltage. Current and voltage readings were takenevery 24 hrs before and after adjusting the current reading back to theoriginal value.

The five-day electrokinetic treatment period was conducted with aconstant voltage application. Small drifts in the applied currentrequired daily voltage adjustments. These adjustments tended to be largedue to the high resistance of the circuit. The current for most partremained stable. As a result, the circuit parameters showed an increasevoltage over time. The expectation is that the voltage requirementshould go up significantly as the reaction progresses. In generalelectrode polarization tends to cause current drop. In this case, theongoing reaction removes ions and nanoparticles from solution. Theseions are the carriers of the electric current. They react and no longerremain as ions as the reaction progresses. This causes resistance todrop, leading to an increase in voltage.

The maximum compressive load test was conducted to determine the changein the load resistance of the block after the treatment. The impact onload resistance in each Set of blocks is also shown in Table 1.

TABLE 1 Compression test. Light Weight Specimen Heavy Weight SpecimenIncrease in Increase in Failure Load Failure Load Load Resistance LoadResistance Specimen (lbs) (%) Specimen (lbs) (%) LCB1 7200 119 HCB113600 101 LEB1 15800 HEB1 27400 LCB2 9900 134 HCB2 14800 105 LEB2 23200HEB2 30300 LCB3 9600 138 HCB3 16100 110 LEB3 22800 HEB3 33800The percentage of increase in load resistance in light weight blocks wasgreatest for specimen LEB3 with a value of 138% as compared to 134% forspecimen LEB2 and 119% for specimen LEB1. The average increase in loadresistance among the light weight blocks was 130%. The maximum increasein load resistance in heavy weight blocks was observed in specimen HEB3with an increase of 110%. The average increase in load resistance forthe heavy weight blocks was 109%.

Current was supplied at the density of 9 mA/m² in order to minimizedamage to the blocks. For a treatment surface area of 0.24 m² theresultant current was 38 mA. This was the applied current for specimenLEB1. It was later taken into account that the masonry blocks could betreated with double the current density since the particles tended toheal the damage from the current. This observation enabled the use of acurrent of 76 mA for blocks from Set 2 and Set 3. This healingassumption was demonstrated by the compressive load resistance observed.The load resistance increase in light weight blocks was higher forspecimens LEB2 and LEB3 with an average increase of 136% as compared toa 119% increase for specimen LEB1. This demonstrated a difference ofonly 19%. This behavior also suggests that the different currentdensities did not produce a significant difference in result and thatfive days may not be required to achieve a given level of blockperformance.

There was a significant enhancement in the load resistance due totreatment especially in view of the largeness and the structure of themasonry block. The average enhancement in load resistance was increasedby 130% for light weight blocks and by 109% for heavy weight blocks.(Refer to Table 1). A possible reason why this large effect was notfully expected is because the defect probability of a system increaseswith the sample size. For systems of smaller size the effect ofstrengthening would be expected to exhibit better results since thedefect probability would likely be relatively low.

Visual inspection showed that the fracture surface of the masonrystructure exhibited a white precipitate at various locations over thesurface. This formation indicated the presence of a treatment product(probably C—S—H). This product appears to be associated with thedecrease in permeability and the enhancement in load resistance observedin this study.

A very close observation of Table 1 indicates that the average loadresistance of the treated light weight block was 39% greater than theuntreated heavy weight block. This demonstrates that the load resistanceof the heavy weight block was exceeded by electrokinetic treatment ofthe light weight block. This indicates that the treated light weightblock could be used in place of the heavy weight block, which isgenerally 20% greater in mass.

It is also noteworthy that there was a significant reduction in thepermeability of the blocks. The average reduction in permeability forall the treated light weight blocks was 900%. The maximum reduction inpermeability for light weight blocks was observed in Set 2 with areduction of 1600%. For the heavy weight blocks the maximum reduction inpermeability was for Set 3 with 3000% and the average reduction for alltreated blocks was 2200%. It is clear from these results that themixture of ions and suspended silica particles in sodium silicatereacted sufficiently with calcium hydroxide to block the pores of themasonry block thereby bringing about a significant reduction inpermeability.

Example 2

Thermal shock is a severe condition that military aircraft pavementsexperience. Normal strength concrete loses 10-20% of its originalcompressive strength when the temperature is increased to 300° C. It canlose up to 60% of its strength when heated to 600° C. To address thisproblem, specimens were tested that consisted of mixtures of Type IPortland cement. Cylindrically shaped specimens were made from Portlandcement paste and cast into polyethylene vials. The specimens were 50.8mm in diameter and 50.8 mm in height. Nanoparticle treatment commencedimmediately following batching. Two batches of specimens were made withwater/cement (w/c) ratios of 0.4, and 0.5. The Type I Portland cementwas manufacture by Lonestar Industries Inc. An electrokinetic drivecircuit was connected similar to that shown in FIG. 3. Three specimensfrom each batch of nine were treated electrokinetically with 2 nmcolloidal alumina nanoparticles for 14 days. The colloidal alumina waspoured gently at the top of each Portland cement specimen. This pond wasreplenished daily. The current was set to provide a current density of1.1 A/m². The power supply was set in current control mode. During thisperiod, control specimens were stored in limewater. After 14 days, boththe treated and control specimens were placed in a furnace and heatedfor 36 hrs at 550° F. (288° C.). The specimens were removed from thefurnace and water quenched. After quenching, the specimens were cappedwith a sulfur capping compound. These specimens were tested incompression.

Cracks appeared on both the treated and untreated specimens afterquenching following a thermal exposure at 550° F. (288° C.) for 36hours. Compressive tests were conducted on specimens in accordance withASTM standard C150 in the following categories.

1) Non treated and unheated,

2) Non treated and oven tested, and

3) Treated and oven tested specimens.

The strength values are presented in Table 2. These values provide acomparison of the above mentioned cases for the 0.4 and 0.5 w/c ratios.The load application time in each case was 70 s. Each value representsan average of three specimens. It was observed that the unheatedcontrols were stronger than the specimens that were heated and quenched.The specimens that were nanoparticle treated prior to heating andquenching were stronger than the untreated cases as well as thecontrols.

TABLE 2 Compressive Strength Test Values Failure Stress Failure Stressfor 0.5-w/c for 0.4-w/c Specimens Specimens Treatment (MPa) (MPa)Untreated and 15.7 16.2 Unheated Controls Untreated and 11.7 12.5 HeatedTreated and 16.4 27 Heated

Quenching produced cracks on all the specimens. For this reasoncompression testing was needed to discern the level of thermal damage.Table 2 contains a summary of the compression test results. Specimensfrom the 0.5 w/c batch that were heated to 550° F. (288° C.) for 36 hrs(and quenched) exhibited a 26% reduction in compression strength ascompared to the control specimens. Those specimens that were treatedwith nanoparticles exhibited a 5% increase in strength compared to thecontrol specimens. When compared to the untreated and heated specimens,the nanoparticle treated specimens exhibited a strength increase of 40%.This work indicated that the nanoparticle treatment, applied to the 0.5w/c ratio case, provided significant resistance to compressive strengthdegradation that would otherwise result from this thermal exposure.

A second batch of specimens having 0.4 w/c ratio was also prepared andtested. In this batch, the untreated and heated specimens lost 22% ofthe compressive strength compared to the control specimens. If thetreated and heated specimens were compared with the control specimens,the treated and heated specimens exhibited 66% more strength. Treatedand heated specimens exhibited 116% more strength as compared to theuntreated and heated specimens.

Normally the lower the w/c ratio the higher the strength of thespecimens. Results from Table 3 show that the strength of the controlspecimens of 0.4 w/c increased by an average value of 4% compared to the0.5 w/c specimens. In the untreated and heated case the strength of the0.4 w/c specimens was increased by 7%. For the treated and heatedspecimens this w/c-related strength increase was 64%. This indicatesthat by using this nanoparticle treatment the compressive strength ofthe specimens increases even after severe thermal exposure.

All the specimens cracked after quenching from 550° F. (288° C.). Fromthe outside, both the treated and untreated looked similar. Compressivetests indicated that the thermal cracking may have occurred adjacent tothe surface of the specimen but not deep within the core. The corestrength may have played a role in restricting the thermal cracking.Quenching caused high thermal gradients on the surface of the both thetreated and untreated specimens. Due to these gradients, high thermalstresses were expected on the surface of the specimens. When the thermalstresses exceeded the tensile strength of the specimens then crackingwould be expected to occur.

A thermal analysis model was developed using ANSYS (Release 7.0)software to assess the possible extent of thermal cracking induced byquenching. A sequential analysis was done on both treated and untreatedspecimens for calculating thermal stresses. The thermal analysis resultswere compared with the experimental results. The properties of both theuntreated and treated specimens are presented in Table 3. This tableconsists of handbook values adapted for constructing a thermal computermodel analysis of how the concrete responded to being quenched. Themodel indicates how far the thermal cracking reaches and thisinformation was used to determine the strength of the uncracked interiorreferred to as the core region or core strength.

TABLE 3 Properties of Untreated and Treated Specimens PropertiesUntreated Treated Modulus of 26 64 Elasticity (GPa) Poisson's 0.17 0.27Ratio Density 1770 2100 (kg/m3) Thermal 2.8 0.14 Conductivity (W/m-K)Specific Heat 950 983.9 (J/kg/K) Coefficient of 20 × 10-6 11.7 × 10-6Thermal Expansion(/K)

A plot was made of thermal stresses vs. radial distance from the centertowards the outer surface in the untreated case as shown in FIG. 7. Asimilar plot was developed for the treated case as shown in FIG. 8.These stresses were calculated using ANSYS (finite element software).Thermal stresses were calculated to predict the extent of tensilestresses that could cause cracking on the surface of a specimen. Thestress curve plotted assumed no surface cracking but may possiblyindicate the depth of surface cracking. The depth of possible surfacecracking was indicated by the level of stress which exceeds the typicaltensile strength range of 3.6 to 7.2 MPa.

A calculated modulus of elasticity (64 GPa) provided thermal stressesnearer to the tensile strength of the material than was provided usingthe literature value of 413 GPa. The typical level of tensile strengthis in the range of 3.6-7.2 MPa. The stresses obtained were approximately5 times this stress level. Contrarily when the reported modulus ofelasticity (413 GPa) was used for the analysis, the stresses it inducedwere up to 100 times the tensile strength of the material. Based onthese calculations, it appears that the modulus of elasticity had a highpositive impact on the stresses calculated in the specimens. In futurework, a better estimate of this modulus should be obtained.

The finite element analysis results showed that the temperature gradientwas higher for the treated specimens as compared to the untreatedspecimens. The higher thermal gradient is expected as a result of a lowheat transfer rate, but the important thing here is that a higherthermal gradient can provide higher thermal stresses. High displacementsin treated specimens were observed as compared to the untreatedspecimens especially at the outer surface. The elevated coefficient ofthermal expansion for the treated cases (˜2× higher) is causing highdisplacements.

The thermal stress profiles due to quenching indicate a high stressstate from the surface inward reaching 4 mm for the untreated case and 2mm for the treated case. Since tensile strengths for these materialstypically range from 3.6 MPa to 7.2 MPa, it is not likely that theactual stresses attained these values. Thermal cracking clearly provideda great deal of stress relief adjacent to the surface. Deeper within thespecimen cores the model predicted high tensile stresses at or above thematerial tensile strength range. Since the remaining compressivestrengths observed for these heated specimens were fairly high (seeTable 2), it is clear that the tensile stresses predicted in the modelwere not attained. Revisiting the cracked region adjacent to thesurface, one may take the 4 mm and 2 mm ranges of predicted highstresses, as demarcations of the beginning of the uncracked core inthese cases. FIG. 9 contains an illustration of these cores. These coresizes were used to estimate the remaining core strength.

Thermal analysis results indicated that the crack penetration in theuntreated case is up to 4 mm into the surface, whereas in the treatedcase it is only up to 2 mm. The stress in the core was calculated bytaking the product of the apparent diameter stress and apparent diameterarea and dividing by the core area. The apparent and core stress valuesfor the untreated and treated cases are presented in Table 4. Theestimated core strength of the treated specimens was increased by 17%compared to the case of the 0.5 w/c (the mass ratio of water to cementpowder used to create the paste). A high value yields a very porous orweak hardened cement paste and thus a week concrete. 0.5 is considered amoderate value for the w/c ratio.

TABLE 4 Apparent and Core Stress Values 0.5 w/c Ratio 0.4 w/c RatioS_(app) S_(core) S_(app) S_(ore) Case (MPa) (MPa) (MPa) (MPa) Untreated11.7 16.5 12.5 17.6 Treated 16.4 19.3 27 31.8

The impact of the w/c ratio on the response to the nanoparticletreatment was significant. The 0.4 w/c ratio specimens responded with65% more strength than 0.5 w/c ratio cases. From Table 2, experimentalresults showed that the compressive strength of the treated specimensafter thermal exposure increased an overall average of 70% but theincrease in resistance to thermal cracking was not clearly demonstrated.

Furnace test results showed both the treated and untreated specimenscracked at 550° F. (288° C.) following quenching. This work indicatedthat the nanoparticle treatment, applied to the 0.4 w/c and 0.5 w/cratio cases, provided significant resistance to compressive strengthdegradation after thermal exposure. The results showed that there was anincrease in compressive strengths at 550° F. (288° C.) for the treatedcases. Finite element analysis indicated a crack-inducing tensile stressin untreated specimens up to a 4 mm depth into the surface whereas inthe treated case it is only up to 2 mm. The estimated, uncracked corestrength of the treated specimens exhibited an average value that was50% higher than that of the untreated specimens. The Impact of w/c ratioon the response to the nanoparticle treatment was significant with the0.4-w/c ratio specimens responding with 65% more strength than the0.5-w/c ratio cases.

1. A method of strengthening cementitious concrete comprising: a.providing a section of cementitious concrete having a depth; b. placinga substantially inorganic nanoparticle carrier liquid in contact with atleast a first surface of said concrete section, said substantiallyinorganic nanoparticle carrier liquid comprising nanoparticles in acolloidal suspension; c. inducing a current across said concretesection; d. applying said current at sufficient magnitude and forsufficient time that said nanoparticles in said nanoparticle carrierliquid migrate through at least one quarter of said depth of saidconcrete section.
 2. The method according to claim 1, wherein saidnanoparticles are chosen from at least one of silica, alumina, silicate,or aluminate.
 3. The method according to claim 1, wherein said depthextends from said first surface of said concrete section to at least onesecond, opposing surface of said concrete section.
 4. The methodaccording to claim 1, wherein said current has a density of about 10 toabout 1000 mA/ft².
 5. The method according to claim 1, wherein saidcurrent is applied for at least about 12 hours.
 6. The method accordingto claim 1, wherein said current is induced by a first electrodepositioned in contact with said nanoparticle solution.
 7. The methodaccording to claim 6, wherein at least one second surface of saidsection is placed in contact with a conducting liquid and at least onesecond electrode is positioned in said liquid.
 8. The method accordingto claim 7, wherein said conducting liquid is an electrolyte solution.9. The method according to claim 6, wherein at least one secondelectrode is formed by reinforcing metal positioned in said concretesection.
 10. The method according to claim 1, wherein said concretesection is in a substantially cured state.
 11. The method according toclaim 1, wherein said concrete section is in a substantially uncuredstate.
 12. The method according to claim 6, wherein said first electrodeis a wire mesh, or conductive fabric, or a fabric rendered conductivevia wetting with a conductive medium.
 13. The method according to claim1, wherein a compressive strength of said concrete section is increasedby at least about 50 percent.
 14. The method according to claim 1,wherein a tensile strength of said concrete section is increased by atleast about 25%.
 15. The method according to claim 1, wherein a modulusof rupture of said concrete section is increased by at least about 25%.16. The method according to claim 1, wherein said nanoparticles undergoa pozzolanic reaction within said concrete section.
 17. The methodaccording to claim 1, wherein at least one porous material is positionedagainst said surface of said concrete section and said porous materialbrings said nanoparticle solution into contact with said concretesection.
 18. The method according to claim 17, wherein at least oneporous material is positioned against at least one second surface ofsaid concrete section.
 19. The method according to claim 18, whereinsaid porous material is flexible.
 20. The method according to claim 19,wherein said flexible, porous material is a sponge.
 21. The methodaccording to claim 1, wherein said concrete section is renderedsubstantially impermeable.
 22. The method according to claim 1, whereinsaid concrete section is a masonry block.
 23. The method according toclaim 22, wherein said masonry block includes two hollow sections.
 24. Astrengthened concrete section produced by the steps comprising: a.providing a section of cementitious concrete having a depth; b. placinga substantially inorganic nanoparticle carrier liquid in contact with atleast a first surface of said concrete section, said substantiallyinorganic nanoparticle carrier liquid comprising nanoparticles in acolloidal suspension; c. inducing a current across said concretesection; d. applying said current at sufficient magnitude and forsufficient time that said nanoparticles in said nanoparticle carrierliquid migrate through at least one quarter of said depth of saidconcrete section.
 25. The strengthened concrete section of claim 24,wherein said strengthened concrete section is rendered substantiallyimpermeable.